A video content in stereoscopic 3D can be seen if a single image is shown for each eye. The differences between both images are called “disparities” which are the slight differences in the points of view.
Given the manner stereoscopic video content are displayed, spectators have to decouple both physiological systems called accommodation and convergence. As a matter of fact, on-screen disparities stimulate the convergence system while the eye accommodation of observer must keep a state to preserve the depth of field around the screen plane in order for vision to remain sharp.
In order to see an object in stereoscopy 3D, spectators have to determine an observation distance on which disparities will be scaled. Thus, observers of stereoscopic content can take into account at least two pieces of information from the visual sequence so as to perceive the simulated depth: convergence which is associated to the disparity signal and accommodation which is associated to the spatial frequencies on the screen. However, the signal of accommodation tends to be predominant in the elaboration of the scaling distance of binocular disparity. This leads to a perceived depth different from the proposed depth.
It corresponds to an over-estimation of depth for crossed disparities (depth in front of the screen) and under-estimation for uncrossed disparities (depth behind the screen). This phenomenon is further described in the document by Watt et al. (2005). Focus cues affect perceived depth. Journal of Vision, 5(10):7, 834-862.
This phenomenon associated to accommodation directly impacts the quality of 3D experience since the 3D space is perceived as distorted. This effect has also been described as corresponding to the “roundness factor” which corresponds to the object width Dx divided by the object depth Dz for a round object. In FIG. 3, for example, the “roundness factor” of the represented element having a negative crossed disparity is 2. It measures how much the object proportions are affected by the distortion. A perfect roundness factor of 1 indicates that no distortion is perceived.
The graph of FIG. 4 displays for a given observer the estimation of the depth Dz of stereoscopic cylinders relative to their width Dx and simulated distance Dvirtual of the observer from the screen plane. X axis represents the stereoscopic (simulated) distance of the cylinder to the observer, Y left axis represents the observer estimation of the cylinder depth and Y right Axis represents the ratio estimated object depth/actual object width. The graph also shows two linear regressions: the first (1) is the regression line for dots representing the estimation of the cylinder depth, the second (2) is for crosses representing the ratio estimated depth on actual width of the cylinder. These regression lines can be used for adjusting stereoscopic content as long as they provide the transform which is operated by the visual system of the observer. The width of the cylinder is always equal to 70 pixels. The magnitude of disparity required to perceive each cylinder increases with the simulated distance. The depth of the cylinder is therefore more and more over-estimated when the cylinder approaches the observer (in front of the screen) and under-estimated when the object goes away (behind the screen). Thus, a distortion of the cylinder appears.
The correction of this distortion is not solved currently. There is no proposition to provide a correct stereoscopic perception.